As a consequence of the increasing demand for direct reduced iron (DRI), also known as sponge iron, as well as for pre-reduced iron or the like, in steel making procedures, DRI producers have faced the necessity to increase production, preferably by increasing the production rates of their existing facilities. Typical DRI production facilities are disclosed in U.S. Pat. Nos. 3,748,120; 3,765,872; 3,905,806; 4,099,962; 4,150,972; and 4,556,417. The content of these and the other patent documents cited herein are incorporated by reference.
In order to achieve an increase in the production rate of an existing facility, the most important factors to consider are the reduction reactor capacity and the available reducing gas supply.
Focusing on the available reducing gas supply, increasing the reformer capacity is the first option that one might visualize as a solution to the problem. However, it also would be very expensive.
U.S. Pat. No. 5,407,460 to Bueno et al. describes a process specifically directed to increasing plant capacity without modification of an existing reformer. In this process, natural gas (with an oxidant) is passed through a gas reformer, optionally heated, and mixed first with a stream of preheated air and/or O.sub.2 and then with an stream of natural gas. This patent teaches the necessity of a gas pre-heater to treat the O.sub.2 -containing gas stream prior to its combination with the reducing gas. There is no identification of the "oxidant" used in combination with the natural gas fed to the reformer nor is there any specific disclosure of any recycle gas (whether passing through the reformer or otherwise, although reference is made in the example in column 5 to certain prior art, which includes U.S. Pat. No. 4,046,557 and others, but without identifying which embodiment thereof). The hot O.sub.2 -containing gas addition burns some of the reducing gas (and possibly some of the natural gas), thereby producing an increase in the temperature (which increases the kinetics of the reduction reactions), with the result that the reduction reactor is able to produce at higher rates since the residence time required to give the same metallization of iron is shorter. The air addition is limited by agglomeration considerations at the resulting higher temperatures. The natural gas combined with the reducing gas from the reformer is itself reformed by oxidants in the recycle gas when in the presence of newly formed DRI as a catalyst (thus increasing the amount of reducing gas without an increase in the reformer size).
U.S. Pat. No. 5,437,708 to Meissner et al. describes a process wherein after cooling/dewatering in a scrubber unit a first portion of the effluent gas from the reduction reactor is reheated and combined with steam and natural gas in a reformer, with the resulting upgraded reducing gas being cooled to temperatures appropriate for reduction of iron ore by mixing with a directly recycled second portion of the reactor effluent gas. The resulting reducing gas mixture is fed to the reduction reactor. optionally, the recycle gas to the reduction reactor can be enriched with methane via line 47 up to a maximum of 4.5 to 5.0% in the resulting reducing gas mixture fed as recycle bustle gas to the reactor. Although there are air A, methane 46, and steam S sources all clustered near the feed of the direct recycle stream 42 to the reactor inlet 34, there is no air (or steam) injection to direct recycle stream 42 (only optional methane injection 47). Somewhat similarly, see especially FIG. 3 of the aforementioned U.S. Pat. No. 4,046,557.
U.S. Pat. No. 5,064,467 to Dam et al. describes a process without any reformer wherein the effluent gas from the reduction reactor is recycled after being mixed with hot air (or air enriched with oxygen) and make-up natural gas, with the resulting combination being introduced to the reduction reactor as the reducing gas stream. This could be thought of as being the opposite of the Bueno et al patent (with the latter teaching use of a reformer with no direct recycle of reducing gas around the reformer and the other teaching no reformer and thus all the reducing gas being directly recycled), and with neither suggesting any intermediate split recycle. U.S. Pat. Nos. 4,528,030 and 5,110,350 are similar (having no reformer), but omitting the air/O.sub.2 injection.
U.S. Pat. No. 5,387,274 to Dam et al. describes a process similar to the prior reference (U.S. Pat. No. 5,064,467) with the main difference being the adding of natural gas to the discharge zone. See also U.S. Pat. No. 5,078,788 which adapts these latter embodiments to use heavy hydrocarbons in place of natural gas.
The present invention offers significant advantages over the prior art, overcoming the disadvantage of preheating the oxygen containing gas, as well as being able significantly to increase still further the production capacity of an existing reduction plant by providing means for natural gas reforming with directly recycled gas in the reduction reactor, while at the same time utilizing full capacity of the reformer.
A particular objective of the present invention is to increase the reduction capacity of existing DRI production facilities without the necessity of increasing the capacity of their existing gas reformers.
Another object is to provide a method for a better utilization of the effluent gas leaving the reduction reactor in order to use this stream to increase the reducing gas available for producing DRI.
Another object is to provide a process in which is presented an integration of energy with a suitable arrangement of streams in order to increase the reducing capacity of a facility at a low capital cost, and also produce direct reduced iron, sponge iron or the like at low processing costs and high metallization rates.
Other objects and advantages will be pointed out herein or will be obvious to those skilled in the art.